AMPK has been established as a sensor and regulator of cellular energy homeostasis (Hardie, D. G. and Hawley, S. A. AMP-activated protein kinase: the energy charge hypothesis revisited. Bioessays 23: 1112 (2001), Kemp, B. E. et. al. AMP-activated protein kinase, super metabolic regulator. Biochem. Soc. Transactions 31:162 (2003)). Allosteric activation of this kinase due to rising AMP levels occurs in states of cellular energy depletion. The resulting serine/threonine phosphorylation of target enzymes leads to an adaptation of cellular metabolism to the low energy state. The net effect of AMPK activation induced changes is inhibition of ATP consuming processes and activation of ATP generating pathways, and therefore regeneration of ATP stores. Examples of AMPK substrates include acetyl-CoA-carboxylase (ACC) and HMG-CoA-reductase (Carling, D. et. al. A common bicyclic protein kinase cascade inactivates the regulatory enzymes of fatty acid and cholesterol biosynthesis. FEBS Letters 223:217 (1987)). Phosphorylation and therefore inhibition of ACC leads to a decrease in fatty acid synthesis (ATP-consuming) and at the same time to an increase in fatty acid oxidation (ATP-generating). Phosphorylation and resulting inhibition of HMG-CoA reductase leads to a decrease in cholesterol synthesis. Other substrates of AMPK include hormone sensitive lipase (Garton, A. J. et. al. Phosphorylation of bovine hormone-sensitive lipase by the AMP-activated protein kinase. A possible antilipolytic mechanism. Eur. J. Biochem. 179:249 (1989)), glycerol-3-phosphate acyltransferase (Muoio, D. M. et. al. AMP-activated kinase reciprocally regulates triacylglycerol synthesis and fatty acid oxidation in liver and muscle: evidence that sn-glycerol-3-phosphate acyltransferase is a novel target. Biochem. J. 338:783 (1999)), malonyl-CoA decarboxylase (Saha, A. K. et. al. Activation of malonyl-CoA decarboxylase in rat skeletal muscle by contraction and the AMP-activated protein kinase activator 5-aminoimidazole-4-carboxamide-1-.beta.-D-ribofuranoside. J. Biol. Chem. 275:24279 (2000)), some of which are potential drug targets for components of metabolic syndrome. Additional processes that are believed to be regulated through AMPK activation, but for which the exact AMPK substrates have not been identified, include stimulation of glucose transport in skeletal muscle and expressional regulation of key genes in fatty acid and glucose metabolism in liver (Hardie, D. G. and Hawley, S. A. AMP-activated protein kinase: the energy charge hypothesis revisited. Bioessays 23: 1112 (2001), Kemp, B. E. et. al. AMP-activated protein kinase, super metabolic regulator. Biochem. Soc. Transactions 31:162 (2003), Musi, N. and Goodyear, L. J. Targeting the AMP-activated protein kinase for the treatment of Type 2 diabetes. Current Drug Targets-Immune, Endocrine and Metabolic Disorders 2:119 (2002)). For example, decreased expression of glucose-6-phosphatase (Lochhead, P. A. et. al. 5-aminoimidazole-4-carboxamide riboside mimics the effects of insulin on the expression of the 2 key gluconeogenic genes PEPCK and glucose-6-phosphatase. Diabetes 49:896 (2000)), a key enzyme in hepatic glucose production, and SREBP-1c (Zhou, G. et. al. Role of AMP-activated protein kinase in mechanism of metformin action. The J. of Clin. Invest. 108: 1167 (2001)), a key lipogenic transcription factor, has been found following AMPK stimulation.
More recently an involvement of AMPK in the regulation of not only cellular but also whole body energy metabolism has become apparent. It was shown that the adipocyte-derived hormone leptin leads to a stimulation of AMPK and therefore to an increase in fatty acid oxidation in skeletal muscle (Minokoshi, Y. et. al. Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase. Nature 415: 339 (2002)). Adiponectin, another adipocyte derived hormone leading to improved carbohydrate and lipid metabolism, has been demonstrated to stimulate AMPK in liver and skeletal muscle (Yamauchi, T. et. al. Adiponectin stimulates glucose utilization and fatty acid oxidation by activating AMP-activated protein kinase. Nature Medicine 8: 1288 (2002), Tomas, E. et. al. Enhanced muscle fat oxidation and glucose transport by ACRP30 globular domain: Acetyl-CoA carboxylase inhibition and AMP-activated protein kinase activation. PNAS 99: 16309(2002)). The activation of AMPK in these circumstances seems to be independent of increasing cellular AMP levels but rather due to phosphorylation by one or more yet to be identified upstream kinases.
Based on the knowledge of the above-mentioned consequences of AMPK activation, certain beneficial effects could be expected from in vivo activation of AMPK. In liver, decreased expression of gluconeogenic enzymes could reduce hepatic glucose output and improve overall glucose homeostasis, and both direct inhibition and/or reduced expression of key enzymes in lipid metabolism could lead to decreased fatty acid and cholesterol synthesis and increased fatty acid oxidation. Stimulation of AMPK in skeletal muscle could increase glucose uptake and fatty acid oxidation with resulting improvement of glucose homeostasis and, due to a reduction in intra-myocyte triglyceride accumulation, to improved insulin action. Finally, the increase in energy expenditure could lead to a decrease in body weight. The combination of these effects in metabolic syndrome could be expected to reduce the risk for acquiring cardiovascular diseases.
Several studies in rodents support this hypothesis (Bergeron, R. et. al. Effect of 5-aminoimidazole-4-carboxamide-1(beta)-D-ribofuranoside infusion on in vivo glucose metabolism in lean and obese Zucker rats. Diabetes 50:1076 (2001), Song, S. M. et. al. 5-Aminoimidazole-4-darboxamide ribonucleoside treatment improves glucose homeostasis in insulin-resistant diabeted (ob/ob) mice. Diabetologia 45:56 (2002), Halseth, A. E. et. al. Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations. Biochem. and Biophys. Res. Comm. 294:798 (2002), Buhl, E. S. et. al. Long-term AICAR administration reduces metabolic disturbances and lowers blood pressure in rats displaying feature of the insulin resistance syndrome. Diabetes 51: 2199 (2002)). Until recently most in vivo studies have relied on the AMPK activator AICAR, a cell permeable precursor of ZMP. ZMP acts as an intracellular AMP mimic, and, when accumulated to high enough levels, is able to stimulate AMPK activity (Corton, J. M. et. al. 5-Aminoimidazole-4-carboxamide ribonucleoside, a specific method for activating AMP-activated protein kinase in intact cells? Eur. J. Biochem. 229: 558 (1995)). However, ZMP also acts as an AMP mimic in the regulation of other enzymes, and is therefore not a specific AMPK activator (Musi, N. and Goodyear, L. J. Targeting the AMP-activated protein kinase for the treatment of Type 2 diabetes. Current Drug Targets-Immune, Endocrine and Metabolic Disorders 2:119 (2002)). Several in vivo studies have demonstrated beneficial effects of both acute and chronic AICAR administration in rodent models of obesity and Type 2 diabetes (Bergeron, R. et. al. Effect of 5-aminoimidazole-4-carboxamide-1(beta)-D-ribofuranoside infusion on in vivo glucose metabolism in lean and obese Zucker rats. Diabetes 50:1076 (2001), Song, S. M. et. al. 5-Aminoimidazole-4-darboxamide ribonucleoside treatment improves glucose homeostasis in insulin-resistant diabetic (ob/ob) mice. Diabetologia 45:56 (2002), Halseth, A. E. et. al. Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations. Biochem. and Biophys. Res. Comm. 294:798 (2002), Buhl, E. S. et. al. Long-term AICAR administration reduces metabolic disturbances and lowers blood pressure in rats displaying feature of the insulin resistance syndrome. Diabetes 51: 2199 (2002)). For example, 7 week AICAR administration in the obese Zucker (fa/fa) rat leads to a reduction in plasma triglycerides and free fatty acids, an increase in HDL cholesterol, and a normalization of glucose metabolism as assessed by an oral glucose tolerance test (Minokoshi, Y. et. al. Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase. Nature 415: 339 (2002)). In both ob/ob and db/db mice, 8 day AICAR administration reduces blood glucose by 35% (Halseth, A. E. et. al. Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations. Biochem. and Biophys. Res. Comm. 294:798 (2002)). In addition to AICAR, more recently it was found that the diabetes drug metformin can activate AMPK in vivo at high concentrations (Zhou, G. et. al. Role of AMP-activated protein kinase in mechanism of metformin action. The J. of Clin. Invest. 108: 1167 (2001), Musi, N. et. al. Metformin increases AMP-activated protein kinase activity in skeletal muscle of subjects with Type 2 diabetes. Diabetes 51: 2074 (2002)), although it has to be determined to what extent its antidiabetic action relies on this activation. As with leptin and adiponectin, the stimulatory effect of metformin is indirect via a mild inhibition of mitochondrial respiratory chain complex 1 (Leverve X. M. et al. Mitochondrial metabolism and type-2 diabetes: a specific target of metformin. Diabetes Metab. 29: 6588 (2003)). In addition to pharmacologic intervention, several transgenic mouse models have been developed in the last years and initial results are becoming available. Expression of dominant negative AMPK in skeletal muscle of transgenic mice has demonstrated that the AICAR effect on stimulation of glucose transport is dependent on AMPK activation (Mu, J. et. al. A role for AMP-activated protein kinase in contraction and hypoxia-regulated glucose transport in skeletal muscle. Molecular Cell 7: 1085 (2001)), and therefore likely not caused by non-specific ZMP effects. Similar studies in other tissues will help to further define the consequences of AMPK activation. It is believed that pharmacologic activation of AMPK may have benefits in relation to metabolic syndrome with improved glucose and lipid metabolism and a reduction in body weight. To qualify a patient as having metabolic syndrome, three out of the five following criteria must be met: elevated blood pressure above 130/85 mmHg, fasting blood glucose above 110 mg/dl, abdominal obesity above 40″ (men) or 35″ (women) waist circumference, and blood lipid changes as defined by an increase in triglycerides above 150 mg/dl or decreased HDL cholesterol below 40 mg/dl (men) or 50 mg/dl (women). Therefore, the combined effects that may be achieved through activation of AMPK in a patient who qualifies as having metabolic syndrome would raise the interest of this target.
Lowering of blood pressure has been reported to be a consequence of AMPK activation (Buhl, E. S. et. al. Long-term AICAR administration reduces metabolic disturbances and lowers blood pressure in rats displaying feature of the insulin resistance syndrome. Diabetes 51: 2199 (2002)), therefore activation of AMPK might have beneficial effects in hypertension. Through combination of some or all of the above-mentioned effects stimulation of AMPK may to reduce the incidence of cardiovascular diseases (e.g. MI, stroke). Increased fatty acid synthesis is a characteristic of many tumor cells, therefore decreased synthesis of fatty acids through activation of AMPK could be useful as a cancer therapy (Huang X. et al. Important role of the LKB1-AMPK pathway in suppressing tumorigenesis in PTEN-deficient mice. Biochem J. 412: 211 (2008). AMPK can also be considered as a metabolic tumor suppressor and AMPK activators could be helpful in general cancer therapy (Luo Z. Et al. AMPK as a metabolic tumor suppressor: control of metabolism and cell growth. Future Oncol. 6: 457 (2010)). Pharmacological activation of the LKB1/AMPK/mTOR axis using known AMPK activators such as metformin, AICAR or A-769662 induce in most studies a dramatic suppression of cancer cell growth, demonstrating that the reinforcement of the tumor suppressive functions of LKB1/AMPK is a valuable therapeutic strategy for both solid tumors (such as breast or prostate cancer) and hematological cancers (such as acute myeloid leukemia or chronic myelogenous leukemia) (Green A. S. et al. LKB1/AMPK/mTOR signaling pathway in hematological malignancies: From metabolism to cancer cell biology. Cell Cycle 10: 2115 (2011). Micic D. et al. Metformin: Its emerging role in oncology. Hormones 10:5 (2011)). The connection of AMPK with several tumour suppressors suggests that therapeutic manipulation of this pathway using AMPK activators warrants further investigation in patients with cancer such as Peutz-Jeghers syndrome, a dominantly inherited cancer-predisposition syndrome in which, at least 80% of all reported cases are caused by mutations that inactivate the gene encoding LKB1 (chromosome 19p13.3), AMPK upstream kinase (Shackelford D. B.; Shaw R. J. The LKB1-AMPK pathway: metabolism and growth control in tumour suppression. Nature Rev. Cancer 2009, 9: 563 (2009). Carling D. LKB1: a sweet side to Peutz-Jeghers syndrome? TRENDS in Molecular Medicine 12: 144 (2006)).
Stimulation of AMPK has been shown to stimulate production of ketone bodies from astrocytes (Blazquez, C. et. al. The AMP-activated protein kinase is involved in the regulation of ketone body production by astrocytes. J. Neurochem. 73: 1674 (1999)), and might therefore be a strategy to treat ischemic events in the brain. Stimulation of AMPK has been shown to improve cognition and neurodegenerative diseases in a mice model (Dagon Y. et al. Nutritional status, cognition, and survival: a new role for leptin and AMP kinase. J. Biol. Chem. 280:42142 (2005)). Stimulation of AMPK has been shown to stimulate expression of uncoupling protein 3 (UCP3) in skeletal muscle (Zhou, M. et. al. UCP-3 expression in skeletal muscle: effects of exercise, hypoxia, and AMP-activated protein kinase. Am. J. Physiol. Endocrinol. Metab. 279: E622 (2000)) and might therefore be a way to prevent damage from reactive oxygen species. Endothelial NO synthase (eNOS) has been shown to be activated through AMPK mediated phosphorylation (Chen, Z.-P., et. al. AMP-activated protein kinase phosphorylation of endothelial NO synthase. FEBS Letters 443: 285 (1999)), therefore AMPK activation may be used to improve local circulatory systems. AMPK has also been described to directly affect PGC-1alpha activity through phosphorylation and then regulate mitochondria biogenesis (Jager S, et al. AMP-activated protein kinase (AMPK) action in skeletal muscle via direct phosphorylation of PGC-1alpha. Proc Natl Acad Sci 104:12017 (2007)). AMPK activation can be then a way to treat mitochondrial disorders (e.g. sarcopenia and some mitochondrial rare diseases). Recently, several reports describe beneficial effect of AMPK activation on virus infection. While virus infection is found to reduce AMPK activity in infected cells or tissues, AMPK activation is proposed as a anti-viral therapy (Mankouri J. et al., Enhanced hepatitis C virus genome replication and lipid accumulation mediated by inhibition of AMP-activated protein kinase, Proc Natl Acad Sci 107: 11549 (2010)).
The use of AMPK activators may represent a strategy to protect the heart and other solid organs against cardiac ischemia as it has been demonstrated with A-769662 (Kim A. S. et al. A small molecule AMPK activator protects the heart against ischemia-reperfusion injury. J. Mol. Cell. Cardiology 51: 24 (2011)) or metformin (Yin M. et al. Metformin improves cardiac function in a non-diabetic rat model of 2 post-MI heart failure Am J Physiol Heart Circ Physiol 301: H459 (2011)).